The announcement comes after similar orders were issued in Los Angeles County and city, Palms Springs and multiple counties in the Bay Area.
Coronavirus: Many Southern California businesses closed, empty as residents heed 'Stay at Home' order
"People will self-regulate their behavior and they'll begin to adjust and adapt as they have been like significantly. We will have social pressure to encourage people to do the right thing," Newsom said at a Thursday press conference.
Gov. Newsom also mentioned "bending the curve" multiple times, in reference to keeping the surge of cases in California as low as possible over the coming weeks.
The order, which goes into effect on March 19 until further notice, shuts down all non-essential businesses. Essential services such as gas stations, pharmacies, grocery stores and banks will remain open.
California health officials predict that about 56% of the state's population -- 25.5 million people -- will be infected with the novel coronavirus within the next eight weeks, Gov. Gavin Newsom said in a letter to President Donald Trump.
The letter, dated March 18, echoes a phone conversation with Trump on Tuesday in which Newsom formally requested that the Navy's USNS Mercy, the largest hospital ship in the world, be deployed to California.
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Newsom's new order followed his previous decision to largely allow local government to set their own rules.
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The governor initially set statewide guidance to keep crowds under 250 people, then on Monday night restricted all unnecessary gatherings and ordered restaurants to close to dine-in patrons and gyms and movie theaters to shutter.
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Local government leaders in Los Angeles and the Bay Area had already taken such steps, or even more aggressive ones. Before Tuesday, he never mandated school closures statewide, though almost every district shut down independently.